1. Field of the Invention
The present invention pertains to a well cleanout system comprising plural serially-injected fluid compositions and a method for removing contaminants from cased and open-hole wellbores using such compositions.
2. Background
In the drilling and workover of oil and gas wells and certain types of fluid-injection wells, it is necessary and desirable to remove contaminants such as drilling fluids, pipe sealants and lubricants and other residue from the wellbore. Failure to perform an effective cleanout operation can result in contamination and plugging of an earth formation from which fluids are desired to be produced. Moreover, wells which have already produced subterranean fluids, including wells having so-called open-hole wellbores, periodically benefit from cleaning of the wellbore to remove contaminants such as iron sulfide precipitates, injection water contaminants and filter cake of certain fluids which have been injected into the wellbore but which have not been removed as a result of producing fluids from the wellbore.
The objectives in wellbore cleanout processes are primarily focused on complete displacement of drilling fluids or other fluids occupying the wellbore and removal of drilling fluid residue and other contaminants occupying the wellbore. In this regard, a displacement fluid, sometimes called a spacer fluid, should be similar in density to the drilling or other fluid occupying the wellbore to prevent substantial commingling of these fluids during the displacement process. Still further, the displacement fluid should contain an agent which is effective in removing contaminants adhering to the wellbore walls as well as certain solids which may be loosely in residence in the wellbore.
Still further, an effective drive fluid for driving the displacement fluid through the wellbore is one which will minimize mixing with the displacement fluid and has a "piston" effect for driving the displacement fluid through the wellbore space and out of the wellbore without substantial mixing with the drive fluid. This fluid should have viscoelastic and pseudo plastic time independent properties to provide the piston-like displacement effect and gel-like suspension characteristics.
Still further, a desirable well cleanout system and method requires a buffer to prevent mixing of the viscous gel drive fluid with a final stage comprising a cleaning or wash pill to provide the final stage of cleanout.
Prior art efforts to provide effective well cleanout processes have resulted in unsatisfactory quality and speed of cleanout as well as unwanted mixing and failure of the fluid slugs or pills displaced through the wellbore to provide the cleanout process. However, the objectives and desiderata outlined above have been met with the improved system and method of the present invention.